Biomarkers are biological molecules that can be used as indicators of the physiologic state of a disease, as well as detectors for following changes during disease progression. Therefore, biomarkers can serve an important role in clinical medicine for detection (including early detection before the presence of clinical disease), diagnosis, treatment, monitoring, risk, and prognosis of diseases. With the completion of the initial draft of the human genome and the rapid growth in high throughput assays, the race for discovery of biological markers has begun.
Two significant limitations of biomarkers include the following. First, they are present in low concentrations and difficult to discriminate from noise. Second, most biomarkers are detected in compartments remote from their origin so that the location of the site producing the biomarker is occult (e.g., the biomarker of primary prostate cancer, PSA, is produced in the prostate but is generally detected in the blood). Once the prostate cancer spreads and PSA is elevated, the blood biomarker does not reveal the site of disease spread which has produced the PSA).